Knight of the Reliquary’s first ability applies only while it’s on the battlefield. In all other zones, it’s a 2/2 creature.

11/17/2017

The activated ability’s cost checks the land’s subtype, not its name. You can sacrifice a nonbasic land this way as long as it has the subtype Forest or Plains. Most nonbasic lands don’t have basic land types, even if they produce colored mana. For example, Horizon Canopy is neither a Forest nor a Plains, while Temple Garden is both.

11/17/2017

Sacrificing a Forest or Plains is part of the cost of Knight of the Reliquary’s activated ability. Assuming that sacrificing the land puts it into your graveyard, Knight of the Reliquary’s first ability will immediately give it an additional +1/+1 when that cost is paid because there’ll be a new land card in your graveyard. Paying a cost can’t be responded to (with Shock, for example).

11/17/2017

Knight of the Reliquary’s activated ability lets you find any land card, not just a basic land card.